Provide clients with (optional) diffing notifications — split up IGListAdapterUpdaterDelegate into finer-grained delegates?

Internally, we've come across a case where we need to know the results of the diffing. (Specifically, we need to know what was re-ordered in a list.)

We _could_ using the diffing components separately to figure this out, but that doesn't make sense since `ListAdapter` will be "re-doing" this work.

We could add a `nullable id` (or something similar) that clients could set in order to get (optional) updates on the diffing results (i.e., to receive the `IGListIndexSetResult` from `IGListDiffing()`).

We'd add the hook [here](https://github.com/Instagram/IGListKit/blob/master/Source/IGListAdapterUpdater.m#L168) to pass the `result` to the delegate/listener.